SO2 Phototriggered Crystalline Nanomechanical Transduction of

Article pubs.acs.org/JPCC

SO2 Phototriggered Crystalline Nanomechanical Transduction of Aromatic Rotors in Tosylates: Rationalization via Photocrystallography of [Ru(NH3)4SO2X]tosylate2 (X = pyridine, 3‑Clpyridine, 4‑Cl-pyridine) Sven O. Sylvester,† Jacqueline M. Cole,*,†,‡ Paul G. Waddell,† Harriott Nowell,§ and Claire Wilson§ †

Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge, CB3 0HE, United Kingdom Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States § Diamond Light Source, Harwell Science Campus, Oxon, OX11 0DE, United Kingdom ‡

S Supporting Information *

ABSTRACT: Thermally reversible solid-state linkage SO2 photoisomers of three complexes in the [Ru(NH3)4SO2X]tosylate2 family are captured in their metastable states using photocrystallography, where X = pyridine (1), 3-Cl-pyridine (2), and 4-Cl-pyridine (3). This photoisomerism exists only in the single-crystal form; accordingly, the nature of the crystalline environment surrounding the photoactive species controls its properties. In particular, the structural role of the tosylate anion needs to be understood against possible chemical influences due to varying the trans ligand, X. The photoexcited geometries, photoconversion levels, and thermal stabilities of the photoisomers that form in 1−3 are therefore studied. 1 and 2 yield two photoisomers at 100 K: the Obound end-on η1-SO2 (MS1) configuration and the side-bound η2-SO2 (MS2); 3 exhibits only the more thermally stable MS2 geometry. The decay kinetics of the MS2 geometry for 1−3 demonstrate that the greater the free volume of the GS SO2 ligand for a given counterion, the greater the MS2 thermal stability. Furthermore, a rationalization is sought for the SO2 phototriggered molecular rotation of the phenyl ring in the tosylate anion; this is selectively observed in 2, manifesting as nanomechanical molecular transduction. This molecular transduction was not observed in 1, despite the presence of the MS1 geometry due to the close intermolecular interactions between the MS1 SO2 and the neighboring tosylate ion. The decay of this anionic molecular rotor in 2, however, follows a nontraditional decay pathway, as determined by time-resolved crystallographic analysis; this contrasts with the well-behaved first-order kinetic decay of its MS1 SO2 phototrigger.

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[Ru(NH3)4SO2X]Y complexes: a side-bound η2-SO2 (MS2) and O-bound end-on η1-SO2 (MS1) configuration, relative to the dark ground-state S-bound η1-SO2 (GS) isomer (Figure 1).2,8 MS1 is less thermally stable than MS2; as such, kinetic photocrystallographic studies have shown that MS1 thermally decays into MS2 before returning to the GS when exposed to higher temperatures.3,7 The existence of one or more of these photoisomers, their photoconversion fractions, their decay temperatures, and the associated rate equations and rate constants are all sensitive to the nature of the ligand, X, that lies trans to the photoisomerizable SO2 in the ruthenium cation.7 MS1 and MS2 photoconversion fractions have been found up to 44.5(7) and 57.6(9)%, respectively, while the highest reported decay temperatures (140 K for MS1) in [Ru-

INTRODUCTION

Solid-state ruthenium sulfur dioxide linkage photoisomerism has been studied in a range of [Ru(NH3)4SO2X]Y complexes (X = trans ligand; Y = counterion(s)) using the developing technique of photocrystallography.1−8 This technique9−12 has proven to be invaluable in the study of such materials where the photoisomerism is not only specific to the solid-state but also dependent on the precise crystal structure arrangement of the complex.13−16 Crystal packing forces, especially intermolecular interactions, are so influential that they can offset chemical substitution effects that one might normally expect to dominate the nature of metal-based linkage photoisomerism.5 This sensitivity to crystallographic environment can be such that entirely different SO2 photoisomer characteristics are observed within the same unit cell where two cations are crystallographically independent of each other.5 Over the past decade, photocrystallography has served to discover and quantify two metastable SO2 photoisomers in © 2014 American Chemical Society

Received: April 15, 2014 Revised: June 30, 2014 Published: June 30, 2014 16003

dx.doi.org/10.1021/jp503711h | J. Phys. Chem. C 2014, 118, 16003−16010

The Journal of Physical Chemistry C

Article

group, P1,̅ so that any crystallographic differences are confined to steric or electronic influences associated with the ions. The MS1 and MS2 photoconversion fractions and kinetic decay characteristics of 1−3 are assessed. In the singular case (2) where molecular transduction occurs, the commensurate levels of SO2 photoconversion in the cation and molecular rotation in the phenyl ring of the tosylate counterion are quantified. The exclusivity of the counterion rotation in 2 is rationalized via comparisons of the SO2 crystallographic environment and metastable thermal stabilities with those of complex 1. The former comparison is enabled via the analysis of Hirshfeld surfaces and Voronoi−Dirichlet polyhedra (VDP) partitioning, focusing on the effects of changing the SO2 reaction cavity.18,19 The latter is explored via kinetic photocrystallographic experiments that follow the thermal decay of each SO2 photoisomer.

Figure 1. Schematic of the crystallographically resolved SO2 groundand photoexcited geometries. The image on the left shows the SO2 in the η1-S-bound ground state (GS), with the other two images showing the two metastable photoinduced geometries: the η2-side-S,O-bound (MS2) and η1-O-bound (MS1) forms.

(NH3)4SO2(pyridine)]Cl2·H2O and (250 K for MS2) in [Ru(NH3)4SO2(H2O)]tosylate2 arise from first-order decay.1,2,7 For over a decade, the counterion in metal-based linkage photoisomers was assumed to play a purely steric role in certain cases, with larger anions permitting higher photoconversion levels and more thermally stable photoisomerization in the ruthenium-based cation.4 Recent work has overthrown this assumption via the demonstration that the counterion can also play an indirect, but very significant, photoactive role through nanomechanical transduction.6 It has been suggested that such molecular transduction could actually be used in the creation of solar-powered molecular machines. This class of materials has also been touted for applications in solid-state optical switches and optical data storage media.10−12 However, if these prospects are to be realized in practical device technologies, then one needs to better understand the delicate interplay between chemical and crystallographic effects that govern their photoisomer characteristics. This leads to the ultimate goal of being able to tailor specifications of these complexes to deliver optical MS1 and MS2 photoconversion fractions that can be temperature-tuned to meet a given device application. Recent progress in understanding chemical effects of the trans ligand has been achieved in cases where the crystallographic environment is essentially kept constant. To this end, it has been shown that the MS2 geometry is primarily stabilized by an increasingly strong Ru−(S,O) σ-bond, i.e., trans ligands with a low pKa value;7 greater stability in this bond corresponds to a weaker trans influence in the trans ligand.17 That study7 was the first to present a rational molecular design protocol for this class of materials. However, where the crystallographic environment varies, this design protocol is no longer adequate when considered alone. In particular, the role of the counterion needs addressing in light of the recent discovery that the nanomechanical effects of the SO2 photoisomerization can be so significant that it can induce counterions to act as a solid-state molecular rotor.6 Such counterion rotors so far discovered are tosylate-based, where its arene group rotates. Accordingly, this work sets out to discriminate the chemical and structural influences on SO2 linkage photoisomerism in this family of complexes where tosylate anions are hosted. It aims to achieve this goal via an examination of the same trans ligand series (pyridine, 3-Cl-pyridine, 4-Cl-pyridine) that was used to establish the aforementioned chemical design protocol, yet substituting the small chloride ion employed there for tosylate anions. Crystals of [Ru(NH3)4SO2X]tosylate2, where X = pyridine (1), 3-Cl-pyridine (2), and 4-Cl-pyridine (3), were therefore studied, with 1 and 3 being synthesized and characterized for the first time. It will be shown that all three complexes are isomorphous and crystallize in the same space

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EXPERIMENTAL METHODS Synthesis of [Ru(NH3)4SO2pyridine]tosylate2 (1). [Ru(NH3)4SO2Cl]Cl (5 mg, 16 μmol), prepared by literature methods,2 was dissolved in a solution of NaCO3 (1.0 mL, 1 M, dissolved in H2O). Pyridine (150 μL) was added to the solution followed by p-toluenesulfonic acid (550 μL, 2 M), which was added dropwise until the solution appeared colorless. Red block-shaped crystals appeared within 2−4 h, which were isolated through vacuum filtration and washed with methanol. A small crystal suitable for X-ray analysis was selected from the solid. Synthesis of [Ru(NH3)4SO23-Cl-pyridine]tosylate2 (2). [Ru(NH3)4SO2Cl]Cl (5 mg, 16 μmol), prepared by literature methods,2 was dissolved in a solution of NaCO3 (1.0 mL, 1 M, dissolved in H2O), EtOH (500 μL) and 3-Cl-pyridine (150 μL) were added, and the solution turned from orange to yellow. pToluenesulfonic acid (550 μL, 2 M) was added dropwise to the solution. Block-like red crystals appeared within 2−4 h, which were isolated through vacuum filtration and washed with methanol. A small crystal suitable for X-ray analysis was selected from the solid. Synthesis of [Ru(NH3)4SO24-Cl-pyridine]tosylate2 (3). [Ru(NH3)4SO2Cl]Cl (5 mg, 16 μmol), prepared by literature methods,2 was dissolved in a solution of NaCO3 (1.0 mL, 1 M, dissolved in H2O). 4-Cl-pyridine (400 μL) was initially added, followed by p-toluenesulfonic acid (550 μL, 2 M), which was added dropwise until block-like red crystals appeared within 2− 4 h. These were isolated through vacuum filtration and washed with methanol. A small crystal suitable for X-ray analysis was selected from the solid. X-ray Photocrystallography. Diffraction measurements, unless otherwise mentioned, were performed on a Rigaku Saturn 724+ CCD diffractometer equipped with a molybdenum X-ray source (Mo Kα, λ = 0.71073 Å) and accompanying SHINE optics for focusing and collimation. An Oxford Cryosystems CryostreamPlus open-flow nitrogen cooling device was used to cool each sample to 100 K, and a full ground-state (GS) structure determination was initially performed on 1−3; these experiments were conducted in complete darkness to prevent any background photoexcitation. The structures were solved by direct methods and refined by full-matrix least-squares methods on F2 values of all data. Refinements were performed using SHELXL,20 with anisotropic atomic displacement parameters refined for all nonhydrogen atoms whose occupation factor was greater than 0.5. The hydrogen atoms were initially placed in idealized positions 16004



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procedures as to those followed in the laboratory were otherwise employed.

for the ammine and aromatic H atoms with Uiso(H) = 1.5Ueq(N) and 1.5Ueq(C), respectively. A summary of the salient crystallographic details for each GS structure is given in Table 1.

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RESULTS AND DISCUSSION Ground-State Structures. The 100 K ground-state crystal structures of 1−3 were found to be isomorphous, crystallizing in the same space group, P1̅. The Ru2+-coordination bond geometries in the cations are comparable with each other (Table 2). At first sight, this is somewhat surprising given that

Table 1. Selected Crystallographic Parameters for the Ground-State Structures of 1−3 crystal color/shape crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) Z T (K) R1 (observed data) wR2 GOF unique reflns no. of parameters

1 (GS)

2 (GS)

3 (GS)

orange blocks triclinic P1̅ 8.341(3) 12.334(5) 13.147(5) 98.451(5) 99.415(7) 90.322(9) 2 100 0.0566 0.125 1.113 8702 331



Table 2. Selected Bond Lengths (Å) and Bond Angles (Deg) for the Ground-State Structures of 1−3 Ru−S (Å) S−Ofree (Å) S−Ofree (Å) Ru−Npyr (Å) Npyr−Ru−S (deg) O−S−O (deg)

1

2

3

2.115(1) 1.440(3) 1.447(3) 2.125(3) 177.52(8) 115.2(2)

2.113(1) 1.444(2) 1.446(2) 2.122(2) 177.96(6) 115.5(1)

2.115(1) 1.439(2) 1.449(2) 2.124(2) 178.20(6) 115.5(1)

there is a significant difference in the Ru−Npyr bond length across the same three trans ligands (X) in this [Ru(NH3)4SO2X]Y series, which hosts chlorides as its counterions, Y.7 On the face of it, this would suggest that chemical effects that usually affect the cation owing to the trans influence are somehow obscured in 1−3. Other than this trans influence structural perturbation discrepancy, the Ru-based cation bond geometry is similar to all other [Ru(NH3)4SO2X]Y complexes.1,2,4−8 The only notable difference observed in the ground-state geometries of 1−3 is in fact associated with one of the tosylate anions in 3, where the oxygen atoms of the SO3 group are disordered. Such disorder has not been seen in other members of this family, although it is common for sulfoxide-containing crystals.24 Photoexcited State Structures. Following in situ illumination, the photoexcited geometries for each of the compounds were examined (Figure 2). Initially, the GS-only model was introduced into the structural refinement of each compound. However, in all three cases, there was evidence of significant residual electron density whose respective intensities and positions were commensurate with those of the anticipated photoisomers. The photoexcited geometries were subsequently introduced and refined to yield photoconversion levels of 16.5(6)% MS1 and 31.6(11)% MS2 (residual GS of 51.9(9)%) for 1, 36.2(4)% MS1 and 6.9(6)% MS2 (residual GS of 56.9(6)%) for 2, and 41(2)% MS2 (residual GS of 59(2)%) for 3. Furthermore, 2 also displayed rotation of the arene moiety within one of the two crystallographically independent tosylate counterions. This molecular rotation has been observed previously and is attributed to the generation of the MS1 geometry.6 Given its apparent origins, it seems strange that 1, which exhibits a larger level of photoconversion to MS1, presents no such rotation in any of its anions. This suggests that the exact molecular transduction mechanism can involve certain subtleties beyond pure MS1 photoconversion level considerations. As such, SO2 photoisomerization to MS1 seems to initiate molecular transduction only in the neighboring counterion when under rather specific crystallographic conditions. Previous work that examined this molecular transduction found that the interaction between the tosylate counterion and

Photocrystallographic measurements were then performed according to literature procedures.5−7,11,21,22 All experimental conditions that had been used for the GS structure determination were maintained except that the crystal was irradiated in situ via an unpolarized Thorlabs Cold White LED (425−650 nm, Figure S1 in Supporting Information) for 2 h while continuously rotating the crystal (360° ϕ rotation every 15 min) in order to achieve the maximum uniformity of sample irradiation. The LED light was delivered through an optical fiber and lens focusing 0.7 W of photons onto a 1 cm2 area that engulfed the crystal. Following irradiation, an otherwise identical data collection was performed as per the GS data. The GS structure was introduced into the photoinduced data as its starting model, and the metastable geometries were determined from residual electron density peaks. The SO2 atoms in the metastable geometries were refined isotropically. To determine the thermal stability of the metastable geometries, multiple X-ray diffraction data collections were then performed successively at a fixed temperature that was just above the thermal threshold for metastability of a given photoinduced structure.3,7 The decay was monitored via the progressively decreasing occupancy of the metastable geometry associated with each successively measured full data set; each time point in this decay process was determined as being the midpoint of a given data collection, with time starting (t = 0) once the decay temperature was initially reached. The decay of the MS1 geometry for 2 at 110 K and the MS2 decay for 1−3 at 200 K (as well as 190 K for 2) were measured using the same method as described above. The MS1 decay of 2 at 120 K was also studied using the UK synchrotron, Diamond Light Source; data collected there employed the single-crystal diffraction beamline, I19.23 I19 hosts a Rigaku Saturn 724+ CCD diffractometer equipped with a Crystal Logic 4-circle kappa goniometer. The X-ray wavelength was tuned to 0.6889(1) Å for this experiment. The temperature was controlled by a combined helium and nitrogen flow via an Oxford Cryosystems nHelix open-flow cryogenic device. Analogous experimental 16005



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by the greater amount of red at the top of its isosurface shown in Figure 3. Comparing the specific intermolecular interactions around Sfree between 1 and 2 (Figure 3 and corresponding Table insets), a large difference in interatomic separation exists between Sfree to C1 and Sfree to H1, whereas the contact distance of Sfree to C2 and Sfree to H2 is similar between the two compounds. Considering the first of these two shorter intermolecular interactions in 2, compared with that in 1, the S1···H1 interaction is longer than the sum of the van der Waals radii for sulfur and hydrogen (2.72 Å); as such, it will not be structurally influential. Rather, the other short intermolecular interaction, S1···C1, is the most likely cause of the molecular rotation in 2; this interaction is closer than the sum of the van der Waals radii (3.50 Å) and is 0.53(1) Å shorter in 2 than it is in 1. These observations suggest that the molecular transduction occurs when the steric hindrance between this tosylate anion and the MS1 SO2 photoisomer is sufficiently large that it needs to alleviate the crystallographic strain associated with this uncomfortable atomic proximity. MS1 Kinetics of 2. The molecular transduction in 2 was further examined by determining the decay kinetics of its MS1 photoisomer levels and the associated arene ring relaxation in its tosylate anionic neighbor. Multiple diffraction data collections were successively performed at 110 and 120 K to determine the decay of MS1 into the MS2 geometry. At 110 K, the MS1 geometry decayed into MS2 via first-order kinetics (τ1/2 = 12(1) h) with a nonzero asymptote (residual long-lived MS1 level: 19.3(7)%), i.e., similar to the MS1 decay kinetics of the [Ru(NH3)4SO2pyridine]Cl2·H2O complex.7 No discernible change was observed in the occupation of the GS geometry throughout this decay. The rotated arene rings involved in the molecular transduction did not follow the same decay pathway as its neighboring MS1 geometry (Figure 4), with no change in

Figure 2. Photoexcited geometries of 1−3 as well as the counterion that neighbors SO2 are shown: carbon (gray), hydrogen (white), sulfur (yellow), oxygen (red), nitrogen (blue), chlorine (green). For 1 and 2, MS1 and MS2 photoisomers both exist, disordered over two sites for 1. For 3, only the MS2 photoisomer is present. The rotated counterion in 2 is shown in green. For both 1 and 3, the oxygens of the sulfoxide group in the SO2 neighboring tosylate counterion are disordered; such disorder also exists in the GS, so it is not photoactive. These images were generated using OLEX.19

the photoisomerized Sfree and Ofree atoms in the MS1 SO2 isomer induces the molecular transduction.6 This interaction can be visualized by displaying the dnorm function over an isosurface that encloses the MS1 cation and is defined by Hirshfeld partitioning.15 It is clear from these Hirshfeld surfaces that the generation of the MS1 geometry induces a close interaction between the cation and the unrotated form of the counterion: this interaction is closer in 2 than in 1, as evidenced

Figure 3. Hirshfeld surfaces enclosing the MS1 cation for 1 and 2 are shown. The red regions show positive isoenergy (abnormally close atomic contact), white regions show neutral isoenergy, and blue regions display negative isoenergy. It is clear from these surfaces that the counterion is closer to S1 in 2 than it is in 1. Associated intermolecular interaction lengths (shown as inset tables) suggest that the S1···C1 interaction (b) is the most relevant interaction for inducing the molecular transduction in 2. The interactions are all given in Ångstroms. 16006



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been measured. This bears a common thread with the 110 K results. Although the half-life of this ring rotation could not be quantified at 120 K, these results clearly show that it decays and that its decay process occurs over a period exceeding 5 h. MS2 Kinetics of 1−3. The thermal stability of the MS2 geometry in 1−3 was also examined to determine the extent of the trans influence across the pyridine, 3-Cl-pyridine, and 4-Clpyridine series of trans ligands with the tosylate counterion, i.e., to assess possible chemical, as opposed to crystallographic, influences on the MS2 photoisomer characteristics of 1−3. In common with the MS1 decay measurements, multiple X-ray diffraction experiments were successively performed on 1−3, at 200 K, and the photoconversion fraction of the MS2 geometry was plotted against the total data acquisition time after reaching 200 K (Figure 5). The decay profile was found to model well

Figure 4. Decay of the MS1 geometry in 2, first at 110 K and then at 120 K (inset), is shown through multiple successive diffraction experiments collected at each temperature. The associated counterion rotation occupation is also shown. At 110 K, the half-life of MS1 is 12(1) h with a long-lived residual MS1 photoconversion fraction of 19.3(7)%; the coefficient of determination, R2, for the fit is 0.989. There was no discernible change in the occupation of the ring rotation at this temperature. At 120 K, the MS1 geometry decays much faster, with a half-life of 0.82(6) h. Modeling the kinetic decay of the associated counterion rotation at 120 K does not provide a good fit; however, it is clear from these data that it decays and does so slower than the corresponding MS1 geometry (see the Supporting Information).

rotated ring occupation over the first 15 h and then a fluctuating photoconversion fraction. Given that the nicely monotonic MS1 decay characteristics derive from the same refined structural model as for this ring, fluctuation owing to statistical variance is ruled out. Rather, this observation indicates that the rotated ring undergoes a nontraditional decay process. This nontraditional decay could be attributed to knock-on nanomechanical effects from neighboring tosylate ions, i.e., relaxation of one ring leads to the rotation of a neighboring ring. This prospect is consistent with the observed diffuse scattering that blurred out many of the Bragg spots in these data, suggesting that cooperative effects may contribute to this process, although it was not possible to confirm this based on the data in hand. Ongoing work aims to unravel the detailed nature of this diffuse scattering. Because the decay kinetics of MS1 at 120 K were so much faster than at 110 K, a synchrotron X-ray source proved to be necessary for these experiments (Figure 4 and Figure S2 in Supporting Information). It was noticed that MS1 also decayed into the MS2 photoisomer via first-order kinetics with a nonzero asymptote; yet, this decay proceeded much more rapidly than at 110 K, with a half-life of 0.82(6) h and a residual MS1 occupation of 10.8(6)%. In common with the 110 K result, the arene ring rotation photoconverted fraction associated with the MS1-induced molecular transduction does not decay in linear proportion to the decay associated with MS1. Indeed, the time lag in decay between MS1 and the arene ring is sufficient to render a photoconversion fraction of the arene ring that is greater than that of the MS1 photoisomer at certain time and temperature points. Significant fluctuation in the ring photoconversion fraction was also observed after 3 h of monitoring its decay, i.e., after four full diffraction scans had

Figure 5. Decay of the MS2 photoisomer into its GS geometry for 1− 3, where each point of the graph represents a full diffraction scan for determining a triclinic crystal structure. The decay of 2 is captured at 190 K in this time window, whereas decay plots of 1 and 3 were evident only at the elevated temperature, 200 K. This suggests that 2 is the least thermally stable of these three compounds. The associated coefficients of determination, R2, for the three curves are 0.993 for 1, 0.933 for 2, and 0.994 for 3.

via first-order kinetics, which replicated the findings of other members of this family. The half-life of 1 and 3 was determined to be 8.4 and 13.1 h, respectively. However, 2 decayed too fast at 200 K to be modeled on a laboratory diffractometer; accordingly, the decay experiment was repeated for 2 but at 190 K; the corresponding half-life was determined to be 36(1) h. These results provide a contrasting trend of thermal stabilities between the same trans ligand series with two different counterions: tosylate2 (this work) and Cl2·H2O.7 In the series with tosylate counterions, the thermal stability follows the rank order according to the trans ligands: 4-Clpyridine (3) > pyridine (1) > 3-Cl-pyridine (2). This compares with complexes bearing the same trans ligand series but with chlorides as the counterions, where thermal stability follows the order: 3-Cl-pyridine (5) > pyridine (4) > 4-Cl-pyridine (6) (Table 3). This suggests that the chemical effects of the trans ligand are not alone in dictating the thermal stability of the MS2 geometry. Indeed, as noted earlier, the Ru−Npyr bond length in 1−3 displays a surprising uniformity, in stark contrast to 4−6 where this bond length varies according to differences in the trans influence of the pyridyl ligand.7 When considering these bond geometry and MS2 thermal stability factors 16007



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Table 3. Comparison of Different Properties across the Two Different Trans Ligand Series as Presented in This Work and Previous Work Cl2·H2O

tosylate2 trans ligand space group pKa photoconversion (MS1) photoconversion (MS2) MS2 t1/2 at 200 K GS (Ru−Npyr) GS (Ru−Sbound) MS2 (Ru−Npyr) MS2 (Ru−Sbound) MS2 (Sbound−Obound) volume (VDP) Ofree···Hneighboring interaction

1

2

3

4

5

6

pyridine P1̅ 5.14 16.5(6) 31.6(11) 8.4 2.125(3) 2.115(1) 2.095(4) 2.44(1) 1.67(3) 40.0 2.48(1)

3-Cl-pyridine P1̅ 2.84 36.2(4) 6.9(6)

SO2 Phototriggered Crystalline Nanomechanical Transduction of

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